1. Field of the Invention
The present invention relates to fractionation of air by selective adsorption and is particularly concerned with a vacuum swing system wherein separate product fractions are recovered including (1) dry oxygen-enriched gas fraction substantially freed of CO.sub.2 and (2) a high purity dry nitrogen product.
2. Prior Art
The numerous and diverse systems disclosed in patent literature for fractionation of the main constituents of air by selective adsorption are discussed in prior U.S. Pat. No. 4,013,419. In accordance with the invention described in said U.S. Pat. No. 4,013,429 there is recovered by selective adsorption from an ambient air feed a high yield of nitrogen of high purity as well as a separate product gas enriched in oxygen. The nitrogen product obtained, however, still contains practically all of the original water initially present in the ambient fresh air feed stream introduced into the system. The water content of the recovered nitrogen product, depending upon ambient humidity and prevailing temperatures, may generally be in the order of up to 3.5% volume percent.
In the operation according to the present invention a dry nitrogen product of high purity is obtained in good yield.
In the practice of the present invention the air fractionation follows generally the sequence of steps also employed in prior U.S. Pat. No. 4,013,429. These air fractionation steps comprise:
1. An adsorption step wherein ambient air is passed in series through a pretreatment adsorbent column to remove moisture and carbon dioxide and then through a main column containing adsorbent selective in retention of the nitrogen component, while discharging oxygen enriched product gas.
2. A subsequent rinsing step wherein both the pretreatment column and the main column are washed with nitrogen collected from a previous step in the cycle. The air like exit gas during this step, which is dry and free of CO.sub.2, is collected and may be added to the fresh air feed in the next adsorption step.
3. Following the nitrogen rinsing, both the pretreatment column and the main column are evacuated to an intermediate pressure thereby desorbing these columns, and the desorbed nitrogen gas is collected. Part of this nitrogen gas is employed as the rinse gas in a subsequent cycle (step 2).
4. Evacuation of the pretreatment column is continued to lowest pressure in the cycle, while the main column at the intermediate pressure is now brought back to desired adsorption pressure by introduction of part of the oxygen-rich product gas collected from step 1, following which gas flow communication between the main column and the associated pretreatment column is restored, thereby bringing the latter to adsorption pressure.
5. With both the pretreater and main adsorption columns now at desired pressure, the sequence is repeated starting with step 1.